The invention concerns thermoplastic molding composition and more particularly a composition containing a polyamide resin.
The invention is based on the surprising and unexpected finding that the impact performance of a thermoplastic composition containing a polyamide resin is improved upon incorporation therewith of a particular silicone rubber powder. The silicone rubber powder, added at a level of about 1 to 25 percent, relative to the weight of the composition, is characterized in that it contains a mixture of (a) a polydiorganosiloxane and (b) silica.
The art has long recognized and been concerned with improving the impact strength of polyamides. A variety of additives have been suggested and added to polyamides with some improvements being obtained. Many of the additives are elastomeric, for instance, U.S. Pat. No. 3,668,274 disclosed improved impact strength by adding a core/shell polymer containing (a) a core made of a cross linked elastomeric phase and (b) a shell made of a rigid thermoplastic phase which contains amine-reactive moieties. An impact resistant polyamide composition containing a graft product of polybutadiene as a graft substrate and a mixture of acrylate and acrylonitrile and or acrylamide monomers grafted thereon was disclosed in U.S. Pat. No. 4,221,879. Also relevant in this context is U.S. Pat. Nos. 4,167,505; 4,174,358 and 4,584,344.
Of particular relevance in the present context is a paper by R. Buch et al "Silicone-Based Additives for Thermoplastic Resins Providing Improved Impact Strength, Processing and Fire Retardant Synergy". This prior art paper (Dow Corning Corporation) disclosed certain silicone powder resin modifiers products termed RM 4-7081 and RM 4-7051 to be useful in reducing the rate of heat release and the evolution rates of smoke and carbon monoxide of burning plastics. Also disclosed is the impact strength improvement for engineering resins such as polyphenylene ether (PPE) and PPS. Improved impact strength of polyamides has not been reported.
Also related is Canadian Patent Application 2,083,014 which disclosed the silicone rubber powder of the present invention as a component in a composition containing poly(phenylene ether) resin.
It has now been discovered that certain silicone rubber powders, preferably produced in accordance with the procedure disclosed in U.S. Pat. No. 5,153,238 which is incorporated herein by reference, are useful as impact modifier in polyamide compositions. The inventive compositions contain about 1 to 25 percent, preferably 3 to 15 percent relative to the weight of the composition, of the silicone rubber powder. The improvement to the impact strength in the context of this invention refers to improvement in the notched Izod impact strength. Dart impact and unnotched Izod impact strength were not noted to be favorably changed as a result of the incorporation of the silicon rubber powder.
The polyamide matrix resin which is suitable for the preparation of the toughened composition of the present invention is well known in the art and is readily available in commerce. Embraced within the scope of the term are semi-crystalline and amorphous polymeric resins having a number average molecular weight (determined by end-group analysis) of about 5,000 to 30,000 commonly referred to as nylons. Preferably, the molecular weight is in the range of about 8,000 to 20,000. Suitable polyamides include those described in U.S. Pat. Nos. 2,071,250; 2,071,251; 2,130,523; 2,130,948; 2,241,322; 2,312,966; 2,513,606; and 3,393,210, the disclosures of which are incorporated herein by reference. Essentially, the polyamide resin can be produced by condensation of equimolar amounts of a saturated dicarboxylic acid containing 4 to 12 carbon atoms with a diamine, in which the diamine contains 4 to 14 carbon atoms. Excess diamine can be employed to provide an excess of amine end groups over carboxyl end groups in the resulting polyamide. Examples of polyamides include polyhexamethylene adipamide (nylon 66), polyhexamethylene azelamide (nylon 69), polyhexamethylene sebacamide (nylon 610), polyhexamethylene dodecanoamide (nylon 612) and bis(paraaminocyclohexyl)methane dodecanoamide. The polyamide resin can also be produced by ring opening of lactams, for example polycaprolactam and polylauric lactam, and by condensation of .omega.-amino carboxylic acids, for example, poly-11-aminoundecanoic acid. Copolyamides prepared by copolymerization of two or more of the above polymers or their components, may also be used. The preferred polyamides are nylon 6, nylon 66, and copolymers of nylon 6 and nylon 66. Preferably the polyamides are linear and have a melting temperature in excess of 200.degree. C.
The silicone rubber powder of the invention has an average particle size of about 1 to 1000 microns and contains (i) 100 parts by weight (pbw) of a polydiorganosiloxane and (ii) about 10 to 80 pbw, preferably about 20 to 50 pbw of a finely divided silica filler.
The polydiorganosiloxane which is characterized in that its viscosity at 25.degree. C. is about 10.sup.6 to 10.sup.9 centipoise is a (co)polymeric resin having siloxane structural units represented by the general formula ##STR1## wherein R, R' and R" independently denote hydrogen, C.sub.1-10 -alkyl, alkenyl, cycloalkyl radicals or aryl groups, and where p is about 1000 to 8000, preferably about 3000 to 6000 and where the relative weight proportions of n and m are 98.5 to 100:0 to 1.5, preferably 99:1, and where
X denotes a member selected from the group consisting of ##STR2## where R denotes hydrogen, C.sub.1-10 -alkyl, alkenyl, cycloalkyl radicals or aryl groups and where q is 1 to 10.
The organic groups of the polydiorganosiloxane, which may optionally be halogenated, are preferably lower alkyl radicals containing 1 to 4 carbon atoms, phenyl and halogen substituted alkyl radicals. Examples include resins containing dimethylsiloxy units, phenylmethylsiloxy units and dimethylsiloxy units and diphenyl siloxy units. Most preferably, the polydiorganosiloxane contains vinyl group(s) or epoxy group(s) at its chain termination(s) and/or along its main chain. The methods for the preparation of suitable polydiorganosiloxane are well known; a typical method comprises the acid- or base-catalyzed polymerization of cyclic diorganosiloxanes.
The silica filler required in the silicone rubber powder is a finely divided silica selected from among fumed silica and precipitated silica or silica gel. These are well known forms of silica and are readily available in commerce. The suitable silica is characterized in that its surface area is at least 50 m.sup.2 /g, preferably 50 to 900 m.sup.2 /g.
An additional embodiment entails use of treated silica which contains sites bonded to groups X as defined above; the manufacture of treated silica, typically by reacting the silanol groups on the silica surface with about 1 to 2% by weight of an organic alkyl halide compound or an organosilicon halide compound, is known in the art.
Among the suitable compounds, mention may be made of low molecular weight liquid hydroxy- or alkoxy-terminated polydiorganosiloxanes, hexaorganosiloxanes and hexaorganosilazanes.
The procedure for the preparation of the silicone rubber powder has been described in detail in U.S. Pat. No. 5,153,238, the specification of which is incorporated herein by reference. Suitable silicone rubber powder is available in commerce from Dow Corning Corporation under the trademark RM 4-7051 and RM 4-7081.
The preparation of the composition of the invention is carried out following conventional procedures and by use of conventional means such as single, preferably twin screw extruders. Conventional thermoplastic processes are suitable in molding useful articles from the inventive composition.
Conventional additives may be incorporated in the composition of the invention in the usual quantities. Mention may be made of a thermal stabilizer, a mold release agent, a pigment, a flame retarding agent, a uv stabilizer, a hydrolysis stabilizer, a gamma radiation stabilizer and a plasticizer for polycarbonate compositions, as well as fillers and reinforcing agents such as glass fibers.